Methods and apparatus for generating and processing wideband signals having reduced discrete power spectral density components

ABSTRACT

Methods and apparatus for generating and processing wideband signals having reduced discrete power spectral density (PSD) components are disclosed. A wideband signal having reduced discrete PSD components is achieved by generating data symbols responsive to data for transmission, transforming one or more of the data symbols into a frame including one or more orthogonal frequency division multiplexing (OFDM) symbols, selectively inverting the frame responsive to a pseudo-random data sequence, and modulating wideband signal pulses of the wideband signal with the selectively inverted frame.

FIELD OF THE INVENTION

The present invention relates to the field of communications and, moreparticularly, to methods and apparatus for generating and processingwideband signals having reduced discrete power spectral densitycomponents.

BACKGROUND OF THE INVENTION

Ultra wideband (UWB) technology uses base-band pulses of very shortduration to spread the energy of transmitted signals very thinly fromnear zero to several GHz. When properly configured, UWB signals cancoexist with other communication signals in the same spectrum withnegligible mutual interference. The Federal Communications Commission(FCC) has specified UWB signal emission limits for UWB communicationsystems to prevent interference with other communication systems.

The emission profile of a UWB signal can be determined by examining itspower spectral density (PSD). Characterization of the PSD of a“Time-Hopping Spread Spectrum” signaling scheme in the presence ofrandom timing jitter using a stochastic approach is disclosed in anarticle by Moe et al. titled “On the Power Spectral Density of DigitalPulse Streams Generated by M-ary Cyclostationary Sequences in thePresence of Stationary Timing Jitter.” See IEEE Tran. on Comm., Vol. 46,no. 9, pp. 1135-1145, September 1998. According to this article, thepower spectra of UWB signals consists of a continuous component anddiscrete components. When total power is the same, the discretecomponents present higher PSD than the continuous component.

Presently, multi-band orthogonal frequency division multiplexing (OFDM)is being considered for use with UWB communication systems. Inmulti-band UWB communication systems using OFDM, the UWB frequency bandis divided into sub-bands and OFDM modulation is applied to eachsub-band.

There is an ever present desire to increase the communication distancesof communication systems such as multi-band UWB communication systemsusing OFDM. One way to increase communication distance is to increasethe power used for transmissions. To increase transmission power whilestill conforming to the FCC emission limits for UWB signals, it isdesirable to reduce the discrete components so that overall power can beincreased while still conforming to the FCC emission limits. Intraditional communication systems, scramblers are commonly used fortiming recovery and equalization. Therefore, these scramblers may not beefficient and/or effective in reducing discrete PSD components insub-bands of multi-band UWB communication systems using OFDM, e.g., dueto the high pulse repetition frequency (PRF), i.e., about 100 Mbps to500 Mbps, and the time division multiple access (TDMA) frame structureof these systems. Accordingly, improved methods and apparatus forreducing discrete PSD components in sub-bands of multi-band UWB signalsusing OFDM are needed. The present invention fulfills this need amongothers.

SUMMARY OF THE INVENTION

The present invention is embodied in methods and apparatus forgenerating and processing wideband signals having reduced discrete powerspectral density (PSD) components. A wideband signal having reduceddiscrete PSD components is achieved by generating data symbolsresponsive to data for transmission, transforming one or more of thedata symbols into a frame including one or more orthogonal frequencydivision multiplexing (OFDM) symbols, selectively inverting the frameresponsive to a random data sequence, and modulating wideband signalpulses of the wideband signal with the selectively inverted frame.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a OFDM wideband communication system inaccordance with the present invention;

FIG. 2 is a flow chart of exemplary processing steps for transmittingOFDM wideband signals in accordance with the present invention;

FIG. 3 is a flow chart of exemplary processing steps for receiving OFDMwideband signals in accordance with the present invention;

FIG. 4 is a timeline depicting configurations for use in simulations inaccordance with the present invention;

FIGS. 5A, 6A, and 7A are graphs depicting PSD versus frequency forsource data having 0%, 25%, and 40% of a particular data value (e.g., avalue of one (1)), respectively, processed using multi-band OFDM withBPSK modulation in accordance with the prior art;

FIGS. 5B, 6B, and 7B are graphs depicting PSD versus frequency forsource data with 0%, 25%, and 40% of a particular data value,respectively, processed using multi-band OFDM with BPSK modulation andselective inversion in accordance with the present invention;

FIGS. 8A, 9A, and 10A are graphs depicting PSD versus frequency forsource data with 0%, 25%, and 40% of a particular data value,respectively, processed using multi-band OFDM with QPSK modulation inaccordance with the prior art;

FIGS. 8B, 9B, and 10B are graphs depicting PSD versus frequency forsource data with 0%, 25%, and 40% of a particular data value,respectively, processed using multi-band OFDM with QPSK modulation andselective inversion in accordance with the present invention;

FIG. 11 is an illustration depicting the configuration of a scrambler inaccordance with one aspect of the present invention;

FIGS. 12A, 13A, 14A, and 15A are graphs depicting PSD versus frequencyfor source data that is scrambled using a 15 bit linear feedback shiftregister with four substantially correlated seeds, where the source dataincludes no non-payload data;

FIG. 12B is a graph depicting PSD versus frequency for source data thatis scrambled using an LFSR-15 with substantially uncorrelated seeds inaccordance with one aspect of the present invention, where the sourcedata includes no non-payload data;

FIG. 13B is a graph depicting PSD versus frequency for source data thatis scrambled using a 28 bit linear feedback shift register (LFSR-28)with substantially uncorrelated seeds in accordance with one aspect ofthe present invention, where the source data includes no non-payloaddata;

FIG. 14B is a graph depicting PSD versus frequency for source data thatis selectively inverted and scrambled using an LFSR-15 withsubstantially uncorrelated seeds in accordance with one aspect of thepresent invention, where the source data includes no non-payload data;and

FIG. 15B is a graph depicting PSD versus frequency for source data thatis selectively inverted and scrambled using a 28 bit linear feedbackshift register (LFSR-28) with substantially uncorrelated seeds inaccordance with one aspect of the present invention, where the sourcedata includes no non-payload data.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a conceptual representation of an exemplary orthogonalfrequency division multiplexing (OFDM) wideband communication system 100in accordance with the present invention. Functions of one or moreblocks within the illustrated communication system 100 can be performedby the same piece of hardware or module of software. It should beunderstood that embodiments of the present invention may be implementedin hardware, software, or a combination thereof. In such embodiments,the various component and steps described below may be implemented inhardware and/or software.

In general overview, a transmitting apparatus 102 for transmittingsource data selectively inverts and, optionally, scrambles the sourcedata prior to transmission using an OFDM modulation scheme to reduce thediscrete power spectral density (PSD) components of the transmitteddata. A receiving apparatus 104 receives the transmitted data,demodulates the received data using an OFDM demodulation scheme, andreverses the inversion and optional scrambling to recover the originalsource data.

The components of the transmitting apparatus 102 and the receivingapparatus 104 are now described in detail. In an exemplary embodiment,the source data is applied to an optional scrambler 106 that isconfigured to scramble the source data. The scrambler 106 may scrambleall of the source data except repetitive data, e.g., synchronizationwords. In an alternative exemplary embodiment, the source data is notscrambled and the optional scrambler 106 can be omitted.

In an exemplary embodiment, the scrambler 106 scrambles at least aportion of the source data using scrambling words. A table of eightexemplary scrambling words (numbered 0-7) are depicted in Table 1. TABLE1 0: 0 0 0 0 1: 0 0 0 1 2: 0 0 1 0 3: 0 0 1 1 4: 0 1 0 0 5: 0 1 0 1 6: 01 1 0 7: 0 1 1 1The exemplary scrambling words may be logically combined with portionsof the source data, e.g., using an XOR logic circuit (not shown), toscramble the source data.

In an alternative exemplary embodiment, a scrambler such as thosedescribed in proposals to the Institute of Electrical and ElectronicEngineer's (IEEE) standard IEEE 802.15.3a is employed to scramble thesource data. The proposed scramblers use a 15-bit linear feedback shiftregister (LFSR) to generate a pseudo-random binary sequence (PRBS) forthe scrambler. At the beginning of each frame, the LFSR is loaded withpredefined values (seeds), which are referred to herein as initialsettings. Four seeds indexed with a two bit identifier (b₁, b₀) aredefined for selection as the initial setting, which is illustrated inTable 2. TABLE 2 Seed identifier (b₁, b₀) Seed value (x₁₄ . . . x₀) 0, 00011 1111 1111 111 0, 1 0111 1111 1111 111 1, 0 1011 1111 1111 111 1, 11111 1111 1111 111The seed values used for scrambling may be selected from the seed setusing the two bit identifier. The selected seed is then logicallycombined with the source data, e.g., using an XOR logic circuit (notshown), to scramble the source data. The two bit identifier may betransmitted in a packet along with the source data for use in thereceiver 104 to initialize a descrambler 134.

As depicted in Table 2, the seed values are highly correlated (i.e.,only the first two bits of each seed value are unique) and, thus, thepseudo random sequences generated are highly correlated, resulting inline spectra (i.e., discrete PSD components) due to a lack of adequaterandomness. The inventors have recognized that superior results in thesuppression of discrete PSD components may be obtained through the useof uncorrelated seeds. Table 3 depicts an exemplary seed set for usewith the scrambler 106. TABLE 3 Seed identifier (b₁, b₀) Seed value (x₂₇. . . x₀) 0, 0 0100 1100 0000 0101 0001 0000 1110 0, 1 1011 1000 01011011 1001 1101 1010 1, 0 0101 1111 1101 0010 1000 0001 1001 1, 1 00001111 0010 1111 0011 0111 1111In Table 3, there are four seed values and each seed value includes 28bits. The seed values are substantially uncorrelated and, therefore,pseudo random sequences generated using these seed values aresubstantially uncorrelated. The seed set shown in Table 3 is forillustration only and seed sets with seeds having different seed values,more or less seeds, and more or less bits per seed may be employed.Those of skill in the art will understand how to generate suitableuncorrelated seed values for use in a seed set from the descriptionherein.

A mapper 108 is coupled to the scrambler 106 to receive scrambled sourcedata. The mapper 108 generates data symbols responsive to the sourcedata. In an exemplary embodiment, the mapper 108 maps source data bitsto data symbols in a frequency domain. The mapper 108 may use aconstellation mapping scheme such as, by was of non-limiting example,binary phase shift keying (BPSK), quadrature phase shift keying (QPSK),or quadrature amplitude modulation (QAM). The mapper 108 may performadditional known functions such as convolutional encoding, puncturing,and bit interleaving.

A transformer 110 is coupled to the mapper 108 to receive the datasymbols. The transformer 110 transforms the data symbols from thefrequency domain to a frame including one or more OFDM symbols in a timedomain. In an exemplary embodiment the transformer 110 employs aninverse Fourier transform such as an inverse discrete Fourier transform(IDFT) or an inverse fast Fourier transform (IFFT). The transformer 110maps the frame of data symbols onto a set of orthogonal frequencysub-carriers where each symbol is mapped to a different sub-carrier. Inan exemplary embodiment, the transformer 110 takes in N data symbols ata time where N is the number of available frequency sub-carriers to forma frame (e.g., a TDMA frame) of OFDM symbols comprised of pulses thatrepresent a summation of sinusoidal values for all N data symbols (i.e.,N carriers).

An inverter 112 is coupled to the transformer 110 to receive the frameof OFDM symbols. The inverter 112 selectively inverts the frame of OFDMsymbols according to a predetermined inverting function. In an exemplaryembodiment, the inverter 112 selectively inverts the frame byselectively inverting entire frames of OFDM symbols responsive to bitsof a random or pseudo-random number sequence. In an alternativeexemplary embodiment, the inverter 112 selectively inverts the frame byselectively inverting one or more individual OFDM symbols within theframe responsive to bits of a pseudo-random number sequence. Theinverter 112 may be a multiplexer (not shown) that passes either an OFDMsymbol or the inverse of the OFDM symbol, e.g., as inverted by aninverter circuit (not shown), responsive to bits of the random orpseudo-random number sequence. The inverter 112 may invert pulses ofOFDM symbols out of the transformer 110 or may invert waveformsgenerated from the pulses. As used herein, random number generator,random number sequence, and random number may be used to refer to bothrandom and pseudo-random generators, sequences, and numbers.

In an exemplary embodiment, the inverter 112 is coupled to a randomnumber generator 114. The random number generator 114 generates randomnumbers or pseudo-random numbers that are at least substantially evenlydistributed to produce the random or pseudo-random number sequence usedby the inverter 112 to selectively invert the frame of OFDM symbols. Asuitable random number generator will be understood by those of skill inthe art from the description herein.

A wideband transmitter 116 is coupled to the inverter 112. The widebandtransmitter 116 transmits the selectively inverted frame of OFDM symbolsvia an antenna 118. In an exemplary embodiment, OFDM symbol pulsesgenerated by the transformer 110 are smoothed using an interpolationfunction and then up-converted to match the number of frequency carriersin at least a portion of the wideband signal. The wideband transmitter116 then modulates the up-converted frame of OFDM symbol pulses ontowideband pulses of the wideband signal such as UWB pulses of a UWBsignal. The up-converted OFDM symbol pulses of the frame may bemodulated onto wideband pulses over the entire wideband signal or ontowideband pulses within a portion of the wideband signal such as asub-band of a multi-band wideband signal. The wideband transmitter 116may be a UWB transmitter or a multi-band UWB transmitter.

A wideband receiver 120 within the receiving apparatus 104 receives thetransmitted wideband signal through another antenna 122 and processesthe wideband signal to recover the selectively inverted frame of OFDMsymbol pulses. A correlator 124 within the wideband receiver 120correlates the received data to the pulse shape used by the transmittingapparatus 102 to identify wideband signal pulses and convert them todigital pulses. In an exemplary embodiment, the correlator 124 is amatched filter correlator configured to identify and correlate incomingframes of OFDM wideband signal pulses such as OFDM UWB pulses. Thewideband receiver 120 processes the correlated OFDM pulses to recoverthe selectively inverted frame of OFDM symbols. The wideband receiver120 may be an UWB receiver or a multi-band UWB receiver.

A synchronizer 126 is coupled to the wideband receiver 120. Thesynchronizer 126 synchronizes the frames of the OFDM symbol pulses forprocessing to recover the original source data. In an exemplaryembodiment, the synchronizer 126 identifies a beginning of a frame bycorrelating pre-defined preambles with the received OFDM signal pulses.Where entire frames of OFDM symbols are selectively inverted, thesynchronizer 126 may additionally determine if the correlating functionfor a frame produces a maximum positive value (indicating that the framehas not been inverted) or a maximum negative value (indicating that theframe has been inverted). The synchronizer may then generate a positivevalue signal if a maximum positive value is determined and a negativevalue signal if a maximum negative value is determined.

An inverter⁻¹ 128 is coupled to the synchronizer 126 to receive theselectively inverted frame of OFDM symbols. The inverter⁻¹ 128 isconfigured to selectively invert the selectively inverted frame of OFDMsymbols to reverse the selective inversion introduced in the transmitter102 by the inverter 112 and, thus, recover the original frame of OFDMsymbol in the time domain. The inverter⁻¹ 128 reverses the inversionaccording to a predefined inverting function that is based on theinverting function of the inverter 112. In an exemplary embodiment, theinverter⁻¹ 128 is coupled to a random number generator 130 that issubstantially identical to the random number generator 114 described indetail above (and, thus, is not described in further detail here). Theinverter⁻¹ 128 may be a multiplexer (not shown) that selectively passesthe selectively inverted frame (or OFDM symbols within the frame) or theinverse of the selectively inverted frame (or individual OFDM symbolswithin the frame), e.g., as inverted by an inverter logic circuit (notshown), responsive to individual bits within a pseudo random numbersequence generated by the random number generator 130.

The two random number generators 110 and 130 generate identical pseudorandom number sequences. For synchronization, the generators 110 and 130may be configured to start at a common point when the first bit of asequence is transmitted or received, e.g., as identified by thesynchronizer 126. Instead of generating pseudo random numbers when datais ready for transmission (e.g., “on the fly”), a set of random numberscan be generated in advance and stored in an array. The same array iskept in the random number generators 110, 130 in both the transmittingapparatus 102 and the receiving apparatus 104. In an exemplaryembodiment, a random number is generated as an index to the stored arrayand is transmitted for use in establishing synchronization between thetransmitting apparatus 102 and the receiving apparatus 104.

In embodiments where entire frames are selectively inverted, theinverter⁻¹ 128 may selectively invert the frame of OFDM symbolsresponsive to positive and negative maximum values identified by thesynchronizer 126. In an exemplary embodiment, the inverter⁻¹ 128 invertsthe frame when a negative value signal is received from the synchronizer126 and does not invert the frame when a positive value signal isreceived from the synchronizer 126. Thus, when entire frames areselectively inverted, the random number generator 130 can be omitted.This enables the random number generator 114 in the transmitter 102 tobe either a random number generator or a pseudo-random number generator.

A transformer 132 is coupled to the inverter⁻¹ 128 to receive theoriginal frame of OFDM symbols. The transformer 132 transforms theoriginal frame of OFDM symbols from the time domain to data symbols inthe frequency domain that were sent to the transformer 110 in thetransmitter 102.

A demapper 134 is coupled to the transformer 132 to receive the datasymbols. The demapper 134 is configured to generate the original sourcedata responsive to the data symbols. The demapper 134 may use aconstellation demapping scheme such as, by was of non-limiting example,binary phase shift keying (BPSK), quadrature phase shift keying (QPSK),and quadrature amplitude modulation (QAM). The demapper 134 may performadditional known functions such as frequency domain equalization (FEQ),de-interleaving, and Viterbi decoding.

An optional descrambler 136 is coupled to the demapper 134 to receivethe source data. In an exemplary embodiment, the descrambler 136, aftersynchronization, reverses the scrambling introduced to the source databy the scrambler 106 to yield the original source data. The descrambler136 reverses the scrambling according to a predefined descramblingfunction that is based on the scrambling function used by the scrambler106. Where the scrambler 106 is omitted, the descrambler 136 may beomitted.

FIG. 2 depicts a flow chart 200 of exemplary processing steps fortransmitting a wideband signal such as a UWB wideband signal withreduced discrete PSD components. The steps of flow chart 200 aredescribed with reference to the components of FIG. 1. At block 202, thetransmitter 102 receives the source data for transmission.

At block 204, the optional scrambler 106 scrambles the source data. Thesource data may include frames of data including payload data andnon-payload data, e.g., synchronization data. In an exemplaryembodiment, the source data is scrambled according to a predeterminedscrambling function, e.g., using scrambling words. The synchronizationdata may be all one symbol such as all positive (+) 1's. In analternative exemplary embodiment, the source data is not scrambled andthe optional scrambler 106 and block 204 can be omitted.

At block 206, the mapper 108 generates data symbols responsive to thesource data. In an exemplary embodiment, the data symbols are in afrequency domain and are generated from the source data using a BPSK,QPSK, QAM, or other such modulation scheme.

At block 208, the transformer 110 transforms one or more data symbolsinto a frame including one or more OFDM symbols. The transformer 110 isan inverse Fourier transformer such as an IDFT or an IFFT thattransforms the data symbols, which are in the frequency domain, into aframe of OFDM symbols in the time domain. In an exemplary embodiment,the transformer 110 transforms N data symbols at a time into the frameof OFDM symbols where N is the number of sub-carriers in the system.

At block 210, the inverter 112 selectively inverts the frame includingone or more OFDM symbols responsive to a random or pseudo-random numbersequence generated by the random number generator 114. In an exemplaryembodiment, the inverter 112 selectively inverts the entire frame ofOFDM symbols responsive to a single bit of the random or pseudo-randomnumber sequence. In an alternative exemplary embodiment, the inverter112 selectively inverts one or more OFDM symbols within a frameresponsive to a signal bit of a pseudo-random number sequence.

At block 212, the wideband transmitter 116 modulates selectivelyinverted frames of OFDM symbols onto wideband pulses of a widebandsignal such as UWB pulses of a UWB signal. In an exemplary embodiment,the frame of OFDM symbols is modulated onto wideband pulses within atleast a portion of the wideband signal. For example, the data symbol maybe modulated onto wideband pulses of the entire wideband signal or of asub-band of a multi-band wideband signal.

At block 214, the transmitter 102 transmits the wideband signalmodulated with the selectively inverted frame of OFDM symbols from theantenna 118.

FIG. 3 depicts a flow chart 300 of exemplary steps for processing areceived wideband signal in accordance with the present invention. Thesteps of flow chart 300 are described with reference to the componentsof FIG. 1. At block 302, the wideband receiver 120 within the receivingapparatus 104 receives the transmitted wideband signal carrying theselectively inverted frame of OFDM symbols through the antenna 122 and,at block 304, the wideband receiver 120 demodulates the wideband signalto recover the selectively inverted frame of OFDM symbol in the timedomain.

At block 306, the inverter⁻¹ 128 reverses the inversion introduced bythe inverter 108 by selectively inverting the frame of OFDM symbols. Inan exemplary embodiment, the frame is selectively inverted responsive toa pseudo-random number sequence generated by the random number generator130 to recover the original frame of OFDM symbols. The random numbergenerator 130 may be synchronized by the synchronizer 126. In analternative exemplary embodiment, where entire frames of OFDM symbolsare selectively inverted, the frame may be selectively invertedresponsive to minimum and maximum values identified by the synchronizer126 during a correlation function and, thus, the random number generator130 may be omitted.

At block 308, the transformer 132 transforms the frame of OFDM symbol inthe time domain into one or more data symbols in the frequency domain.At block 310, the demapper 134 generates source data responsive to theone or more data symbols. In an exemplary embodiment, the source data isgenerated from the data symbols by reversing the BPSK, QPSK, QAM, orother such modulation scheme used by the mapper 108. At block 312, theoptional descrambler 136 reverses the scramble introduced by theoptional scrambler 106 to derive the original source data. Inembodiments where the source data is not scrambled, the descrambler 136and the step in block 312 are omitted.

Additional implementation details are now provided for the exemplarycommunication system 100 described above with reference to FIGS. 1, 2,and 3. The analysis and simulation details below show that line spectra(i.e., discrete PSD components) is an issue for multi-band OFDM UWBsystems. In addition, the analysis and simulation details below show theimprovements in line spectra achievable using the present invention.

An analysis is now provided of the PSD of a current multi-band OFDM UWBsequence for use in wireless systems such as proposed for IEEE standard802.15.3a set forth by the Institute of Electrical and ElectronicEngineers (IEEE).

In a multi-band OFDM UWB communication systems, a digitally controlledsignal is used that produces random transmissions at multiples of theclock period. This signaling technique can be modeled as shown inequation 1: $\begin{matrix}{{{s(t)} = {{\sum\limits_{n = {- \infty}}^{\infty}{{Re}\left\{ {\sum\limits_{k = {- \frac{N_{s}}{2}}}^{\frac{N_{s}}{2} - 1}{d_{n,{k + \frac{N_{s}}{2}}}{\exp\left( {{{j2\pi}\left( {f_{n} - \frac{k + 0.5}{T}} \right)}\left( {t - {nT}_{s}} \right)} \right)}}} \right\}\quad f_{n}}} \in \left\{ {{{f_{m}\text{:}\quad m} = 1},\ldots\quad,M} \right\}}},{{nT}_{s} \leq t \leq {{nT}_{s} + T}}} & (1)\end{matrix}$where Ts equals the symbol clock period or pulse rate, n is the symbolindex, Ns equals the number of tones or sub-carriers in the symbols, tis the timing, {d^(n,k+N/2)} is an unbalanced binary independentidentically distributed (i.i.d.) random sequence, and {f_(m)} is thecenter frequency of each sub-band.

If M bands are used and OFDM symbols are transmitted on sub-bands inturns, the waveform of carrier k in band m can be expressed as shown inequation (2). $\begin{matrix}{{{s_{m,k}(t)} = {{\sum\limits_{n = {- \infty}}^{\infty}{{Re}\left\{ {d_{n,{k + \frac{N_{s}}{2}}}{\exp\left( {{{j2\pi}\left( {f_{m} - \frac{k + 0.5}{T}} \right)}\left( {t - {\left( {{nM} + m} \right)T_{s}}} \right)} \right)}} \right\} 1}} \leq m \leq M}},{{\left( {{nM} + m} \right)T_{s}} \leq t \leq {{\left( {{nM} + m} \right)T_{s}} + T}}} & (2)\end{matrix}$

If BPSK modulation is used on each carrier, the waveform of carrier k onsub-band m can be expressed as shown in equation 3. $\begin{matrix}{{s_{m,k}(t)} = {A_{1}{\sum\limits_{n = {- \infty}}^{\infty}{a_{n,m,k}{w_{m,k}\left( {t - {\left( {{nM} + m} \right)T_{s}}} \right)}}}}} & (3)\end{matrix}$where A₁ is the scale coefficient,${{w_{m,k}(t)} = {\cos\left( {2{\pi\left( {f_{m} - \frac{k + 0.5}{T}} \right)}t} \right)}},{and}$${\Pr\left\{ a_{n,m,k} \right\}} = \left\{ {\begin{matrix}{p,} & {a_{n,m,k} = 1} \\{{1 - p},} & {a_{n,m,k} = {- 1}}\end{matrix}.} \right.$

For QPSK modulations, the waveform of carrier k on band m can beexpressed as shown in equation 4. $\begin{matrix}{{s_{m,k}(t)} = {A_{2}{\sum\limits_{n = {- \infty}}^{\infty}{a_{n,m,k}{w_{m,k}\left( {t - {\left( {{nM} + m} \right)T_{s}}} \right)}}}}} & (4)\end{matrix}$where A₂ is the scale coefficient and${{w_{m,k}(t)} \in \left\{ {{{{\cos\left( {2{\pi\left( {f_{m} - \frac{k + 0.5}{T}} \right)}t} \right)} + {\Phi_{l}\text{:}\quad l}} = 1},2,{{{\Phi_{1} - \Phi_{2}}} = {\pi/4}}} \right\}},{and}$${\Pr\left\{ a_{n,m,k} \right\}} = \left\{ {\begin{matrix}{p,} & {a_{n,m,k} = 1} \\{{1 - p},} & {a_{n,m,k} = {- 1}}\end{matrix}.} \right.$

The PSD of the signals in equations 3 and 4 each consist of continuouscomponent and discrete component, which are expressed, respectively, inequations 5. $\begin{matrix}{{{S_{m,k}^{c}(f)} = {\frac{1 - \left( {{2p} - 1} \right)^{2}}{MTs}{{W_{m,k}(f)}}^{2}}}{{S_{m,k}^{d}(f)} = {\frac{\left( {{2p} - 1} \right)^{2}}{({MTs})^{2}}{\sum\limits_{l = {- \infty}}^{\infty}{{{W_{m,k}\left( \frac{l}{MTs} \right)}}^{2}{\delta_{D}\left( {f - \frac{l}{MTs}} \right)}}}}}} & (5)\end{matrix}$The total PSD is the superposition of the two spectra of the waveformsexpressed by equations 5.

From equations 5, it can be seen that the PSD is determined according tofour factors, i.e.,

-   -   W_(m,k)(f)—pulse shape and transmission power of carrier k in        sub-band m;    -   Ts—clock period or pulse rate;    -   p—distribution of a_(n); and    -   M—total number of sub-bands.

When p=0.5, lines in each sub-band due to the discrete components areminimized or removed, thereby minimizing the PSD of each sub-band. Thenew PSD can be expressed as shown in equations 6. $\begin{matrix}{{{S_{m,k}^{c}(f)} = {\frac{1}{MTs}{{W_{m,k}(f)}}^{2}}}{{{S_{m,k}^{d}(f)} = 0},{{- \frac{N_{s}}{2}} \leq k \leq {\frac{N_{s}}{2} - {1\quad{and}\quad 1}} \leq m \leq M}}} & (6)\end{matrix}$

Details regarding aspects of the present invention to reduce thediscrete PSD component of multi-band UWB signals is now provided. Basedon the preceding analysis of the PSD of multi-band UWB signals,selective phase reversion (inversion) is proposed to reduce/eliminateline frequencies due to discrete PSD components.

First, a random sequence {b_(n,m,k)} is generated. The random sequenceincludes an evenly distributed function represented by equation 7.$\begin{matrix}{{\Pr\left\{ b_{n,m,k} \right\}} = \left\{ \begin{matrix}{0.5,} & {b_{n,m,k} = 1} \\{0.5,} & {b_{n,m,k} = {- 1}}\end{matrix} \right.} & (7)\end{matrix}$

Second, the data sequence and the random sequence are combined, e.g.,using an exclusive OR (XOR) operation on sequences {a_(n,m,k)} and{b_(n,m,k)} as shown in equation 8 to produce a new sequence{c_(n,m,k)}. The new sequence {c_(n,m,k)} is then used for transmission.c _(n,m,k)=a_(n,m,k){circumflex over ( )}b_(n,m,k)  (8)

This process reduces/removes lines (i.e., discrete components) in thePSD of UWB signals in each sub-band, which is equivalent to minimizingthe PSD in each sub-band.

Simulations are now provided to show that the line spectra is still anissue for multi-band OFDM systems without selective inversion. Thesimulation also show that applying selective inversion suppresses linespectra and reduce the PSD of multi-band OFDM UWB signals.

The configuration for the simulations is shown in FIG. 4. Thesimulations use known Periodogram PSD estimators to calculate the PSD ofdifferent UWB signals with only one sub-band used in the simulations.Each OFDM symbol consists of 16 carriers represented by 160 samplesincluding a cyclic prefix followed by 96 samples of zero padding as aguard time. One frame contains 256 symbols. FFT is performed on frames,i.e., 64K-point FFT on 64K samples to evaluate the PSD. Because a singleestimate usually generates a large bias in estimation and the FederalCommunication Commission (FCC) UWB regulations set a limit on averagePSD, 100 runs are used to smooth the final PSD estimate.

Results of the simulations are shown in the graphs of FIGS. 5 to 10depicting PSD versus frequency, with figures designated with a capital Adepicting the PSD generated using original data, e.g., {a_(n,m,k)}, andfigures designated with a capital B depicting the PSD generated usingdata with selective reversion/inversion in accordance with the presentinvention, e.g., {c_(n,m,k)}. FIGS. 5A and 5B, 6A and 6B, and 7A and 7Bprovide a PSD comparison for source data having 0%, 25%, and 40% of aparticular data value (e.g., a value of one (1)), respectively,processed using multi-band OFDM with BPSK modulation. FIGS. 8A and 8B,FIGS. 9A and 9B, and FIGS. 10A and 10B provide a PSD comparison forsource data having 0%, 25%, and 40% of a particular data value (e.g., avalue of one (1)), respectively, processed using multi-band OFDM withQPSK modulation.

The graphs illustrate that in existing multi-band OFDM UWB communicationsystems:

-   -   Line spectra 500 will appear if original data {a_(n,m,k)} is not        evenly distributed regardless of whether BPSK or QPSK is used;    -   Selective phase inversion/reversion performed on original data        can remove/reduce spectra lines. For example, the peak value of        PSD is reduced from about −4.8 dB, −14 dB, and −23 dB to about        −31 dB for BPSK modulation shown in FIGS. 5 to 7, respectively,        and from about −7.8 dB, −17 dB, and −26 dB to about −34 dB for        QPSK modulation shown in FIGS. 8 to 10, respectively; and    -   Regardless of the PSD of the original data, the PSD of processed        data is similar, i.e., −31 dB for BPSK modulation and −34 for        QPSK modulation.

Selective inversion for an IEEE 802.15.3a application is now describedin which selective inversion is applied to frames and is referred to arandom frame reversion (RFR). The IEEE 802.15.3a standard utilizes anIEEE 802.15.3 MAC and the maximal frame length is 2 KByte. Assuming a 16Mbps bandwidth stream is used, the minimal pulse repetition frequency(PRF) of frames is represented by equations 9.16M/(2K*8)=1KT _(f)=10⁻³  (9)

Because T_(f)<<1, non-payload data will generate discrete PSD componentsresulting in strong spectra lines. Table 4 lists values that thenon-payload data contributes to the PSD under the assumption thatT_(f)=10⁻³, where p_(st) is the percentage of non-payload data in a datastream. Table 4 indicates that although non-payload data constitutes asmall portion of a frame, its contribution to PSD cannot be neglecteddue to the high PRF of the frames. If a smaller frame length is used ina harsh environment to reduce frame error rates, the same percentage ofnon-payload data will generate stronger spectral lines and more PSD thanthose listed in Table 4. TABLE 4 p_(st) ΔPSD (dB) 0.1% 3.01 0.5% 7.78  1% 10.41   5% 17.07  10% 20.04

RFR can reduce the discrete PSD component generated by non-payload data,which is not scrambled, and payload data. For simplicity, a framed datastream can be expressed as shown in equation 10. $\begin{matrix}{{s(t)} = {\sum\limits_{l = {- \infty}}^{\infty}{\sum\limits_{k = 1}^{K}{a_{l,k}{w\left( {t - {lT}_{f} - {kT}_{p}} \right)}}}}} & (10)\end{matrix}$where l and k are index values of frames and pulses within the frames.

To reduce the discrete PSD component in an IEEE 802.15.3a system, first,a random sequence {b_(n)} is generated with an evenly distributedfunction as shown in equation 11. $\begin{matrix}{{\Pr\left\{ b_{n} \right\}} = \left\{ \begin{matrix}{0.5,} & {b_{n} = 1} \\{0.5,} & {b_{n} = {- 1}}\end{matrix} \right.} & (11)\end{matrix}$Second, an operation shown in equation 12 is applied to data and therandom sequence to produce a new sequence, {C_(l,k)} for transmission.$\begin{matrix}{c_{l,k} = \left\{ \begin{matrix}{a_{l,k},} & {b_{l} = 1} \\{{- a_{l,k}},} & {b_{l} = {- 1}}\end{matrix} \right.} & (12)\end{matrix}$

Scrambler simulations are now provided. FIG. 11 depicts theconfiguration used for the simulation. In the configuration, the framerate is 128 frames/second and one frame contains 256 symbols. Eachsymbol has 16 carriers that come from the first 16 carriers in the 128carriers that are used in the original multi-band OFDM UWB systems. Thepayload data are all “1”s to the scrambler and there is no non-payloaddata. The simulation results are shown in FIGS. 12A and 12B, where FIG.12A depicts results using the original scrambler (e.g., a fifteen bitlinear feedback shift register with four substantially correlated seeds(see Table 2)) and FIG. 12B depicts results using a fifteen bit linearfeedback shift register with four substantially uncorrelated seedsgenerated by MATLAB rando function. The result show that the scramblerwith four new uncorrelated seeds reduces the PSD by about 16 dB over thescrambler with the four original correlated seeds.

As shown in FIG. 12B, strong spectra lines still exist using a fifteenbit linear feedback shift register with four substantially uncorrelatedseeds. These lines are associated with the relatively short length ofthe linear feedback shift register. To further reduce these lines, alonger polynomial generator, e.g., a twenty eight bit linear feedbackshift register, may be used. The results shown in FIG. 13A are achievedusing an original scrambler, e.g., LFSR-15 with four original correlatedseeds and the results shown in FIG. 13B are achieved using a new 28 bitlinear feedback shift register with four new substantially uncorrelatedseeds. Results in FIG. 12B and FIG. 13B show that using and LFSR-28reduces the PSD by about an additional 3 dB.

Additional simulations are now provided in which the payload data areall “1”s to the scrambler and the non-payload data comprises about 3.1%of the frame. FIG. 14A shown the PSD of data processed by the originalscrambler (i.e., LFSR-15 with four substantially correlated seeds) andFIG. 14B shows the PSD of data processed by LFSR-15 with foursubstantially uncorrelated seeds and RFR. FIG. 15A shown the PSD of dataprocessed by the original scrambler (i.e., LFSR-15 with foursubstantially uncorrelated seeds) and FIG. 15B shows the PSD of dataprocessed by the LFSR-28 using four substantially uncorrelated seeds andwith RFR. The four substantially uncorrelated seeds are generated byMATLAB rando function. Results in FIG. 14B and FIG. 15B show that thenew scheme reduces the discrete PSD components of UWB signalssignificantly, i.e., about 34 dB and 37 dB, respectively, over currentsystems.

The simulation results show that spectral lines due to discrete PSDcomponents are an issue for multi-band OFDM UWB signals. Methods andapparatus are described for base-band processing to reduce/remove thespectral lines, thereby reducing the peak value of the PSD of multi-bandOFDM UWB signals. The simulation results confirm the effectiveness ofthese methods and apparatus in suppressing the discrete components ofthe PSD of multi-band OFDM UWB signals.

Although the components of the present invention have been described interms of specific components, it is contemplated that one or more of thecomponents may be implemented in software running on a computer. In thisembodiment, one or more of the functions of the various components maybe implemented in software that controls the computer. This software maybe embodied in a computer readable carrier, for example, a magnetic oroptical disk, a memory-card or an audio frequency, radio-frequency oroptical carrier wave. The computer may be a general or specific purposecomputer, an application specific integrated circuit (ASIC), statemachine, or essentially any device capable of processing signals asdescribed herein.

Further, although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

1. A method for processing source data for transmission over a widebandsignal such that the wideband signal has reduced discrete power spectraldensity (PSD) components, the wideband signal including wideband signalpulses, the method comprising the steps of: generating data symbolsresponsive to the source data; transforming one or more of the datasymbols into a frame including one or more orthogonal frequency divisionmultiplexing (OFDM) symbols; selectively inverting the frame of OFDMsymbols responsive to a random data sequence; and modulating thewideband signal pulses of the wideband signal with the selectivelyinverted frame of OFDM symbols.
 2. The method of claim 1, wherein thesource data includes bits and the generating step comprises the step of:mapping bits of the source data to the data symbols using one of (i)binary phase shift keying and (ii) quadrature phase shift keying.
 3. Themethod of claim 1, wherein the data symbols are in a frequency domainand wherein the transforming step comprises the step of: transformingthe one or more data symbols from the frequency domain to a time domainto produce the frame including the OFDM symbols.
 4. The method of claim1, further comprising the step of: scrambling the source data prior tothe generating step.
 5. The method of claim 4, wherein the scramblingstep comprises the step of: scrambling the source data using a linearfeedback scheme initialized using substantially uncorrelated seeds 6.The method of claim 1, wherein the selectively inverting step comprisesthe steps of: selectively inverting one or more individual OFDM symbolswithin the frame responsive to the random data sequence.
 7. The methodof claim 1, wherein the wideband signal is an ultra wideband signalincluding ultra wideband signal pulses and wherein the modulating stepcomprises the step of: modulating the ultra wideband signal pulses ofthe ultra wideband signal with the selectively inverted frame of OFDMsymbols.
 8. The method of claim 1, wherein the wideband signal is amulti-band wideband signal including wideband signal pulses and whereinthe modulating step comprises the step of: modulating the widebandsignal pulses corresponding to at least one sub-band of the multi-bandwideband signal with the selectively inverted frame of OFDM symbols. 9.An apparatus for processing source data for transmission over a widebandsignal such that the wideband signal has reduced discrete power spectraldensity (PSD) components, the wideband signal including wideband signalpulses, the apparatus comprising: a mapper configured to generate datasymbols responsive to the source data; a transformer coupled to themapper, the transformer configured to transform one or more data symbolsinto a frame including one or more orthogonal frequency divisionmultiplexing (OFDM) symbols; an inverter coupled to the transformer, theinverter configured to selectively invert the frame of OFDM symbols; anda wideband transmitter coupled to the inverter, the wideband transmitterconfigured to modulate the wideband signal pulses of the wideband signalwith the selectively inverted frame of OFDM symbols.
 10. The apparatusof claim 9, further comprising: a scrambler coupled to the mapper, thescrambler configured to scramble the source data prior to mapping thesource data to the data symbols.
 11. The apparatus of claim 10, whereinthe scrambler comprises: a linear feedback shift register configured forinitialization using seed values; and a memory coupled to the linearfeedback shift register, the memory including a set of seed values forinitialing the linear feedback shift register wherein the seed valuesare substantially uncorrelated with respect to one another.
 12. Theapparatus of claim 9, wherein the wideband transmitter is a multi-bandwideband transmitter.
 13. The apparatus of claim 9, wherein the widebandtransmitter is an ultra wideband transmitter.
 14. The apparatus of claim9, wherein the data symbols are in a frequency domain and thetransformer is configured to transform the data symbols from thefrequency domain into the frame of OFDM symbols in a time domain. 15.The apparatus of claim 9, wherein the inverter is configured toselectively invert one or more individual OFDM symbols within the frame.16. A method for processing a received wideband signal having reduceddiscrete power spectral density (PSD) components to recover source data,the wideband signal including a selectively inverted frame invertedusing a random data sequence, the frame including one or more orthogonalfrequency division multiplexing (OFDM) symbols, the method comprisingthe steps of: de-modulating the wideband signal to recover theselectively inverted frame of OFDM symbols; selectively inverting theframe responsive to the random data sequence to recover an originalframe of OFDM symbols; transforming the original frame of OFDM symbolsinto one or more data symbols; and generating the source data responsiveto the one or more data symbols.
 17. The method of claim 16, wherein theoriginal frame is in a time domain and wherein the transforming stepcomprises the step of: transforming the original frame of OFDM symbolsfrom the time domain to the frequency domain to produce the one or moredata symbols.
 18. The method of claim 16, wherein the source data isscrambled and wherein the method further comprising the step of:descrambling the source data.
 19. The method of claim 16, wherein thewideband signal is an ultra wideband signal and wherein thede-modulating step comprises the step of: de-modulating the ultrawideband signal to recover the selectively inverted frame of OFDMsymbols.
 20. The method of claim 16, wherein the wideband signal is amulti-band wideband signal and wherein the modulating step comprises thestep of: de-modulating the multi-band wideband signal to recover theselectively inverted frame of OFDM symbols.
 21. An apparatus forprocessing a received wideband signal having reduced discrete powerspectral density (PSD) components to recover source data, the widebandsignal including a selectively inverted frame inverted using a randomdata sequence, the frame including one or more orthogonal frequencydivision multiplexing (OFDM) symbols, the apparatus comprising: awideband receiver configured to de-modulate the wideband signal torecover the selectively inverted frame of OFDM symbols; an invertercoupled to the wideband receiver, the inverter configured to selectivelyinvert the recovered selectively inverted frame of OFDM symbols usingthe random data sequence to recover an original frame of OFDM symbols; atransformer coupled to the inverter, the transformer configured totransform the original frame of OFDM symbols into one or more datasymbols; and a mapper coupled to the transformer, the mapper configuredto generate the source data responsive to the one or more data symbols.22. The apparatus of claim 21, wherein the source data is scrambled andwherein the apparatus further comprises: a de-scrambler coupled to themapper, the de-scrambler configured to de-scramble the source data. 23.The apparatus of claim 21, wherein the wideband receiver is a multi-bandwideband receiver.
 24. The apparatus of claim 21, wherein the widebandreceiver is an ultra wideband receiver.
 25. The apparatus of claim 21,wherein the original frame is in a time domain and the transformer isconfigured to transform the original frame of OFDM symbols from the timedomain into the one or more data symbols in a frequency domain.
 26. Asystem for processing source data for transmission over a widebandsignal such that the wideband signal has reduced discrete power spectraldensity (PSD) components, the wideband signal including wideband signalpulses, the system comprising: means for generating data symbolsresponsive to the source data; means for transforming one or more of thedata symbols into a frame including one or more orthogonal frequencydivision multiplexing (OFDM) symbols; means for selectively invertingthe frame of OFDM symbols responsive to a random data sequence; andmeans for modulating the wideband signal pulses of the wideband signalwith the selectively inverted frame of OFDM symbol.
 27. The system ofclaim 26, further comprising: means for scrambling the source data priorto the generating step.
 28. A computer readable carrier includingsoftware that is configured to control a computer to implement awideband signal processing method embodied in a computer readable mediumfor processing source data for transmission over a wideband signal suchthat the wideband signal has reduced discrete power spectral density(PSD) components, the wideband signal including wideband signal pulses,the processing method including the steps of: generating data symbolsresponsive to the source data; transforming one or more of the datasymbols into a frame including one or more orthogonal frequency divisionmultiplexing (OFDM) symbols; selectively inverting the frame of OFDMsymbols responsive to a random data sequence; and modulating thewideband signal pulses of the wideband signal with the selectivelyinverted frame of OFDM symbols.
 29. The computer readable carrier ofclaim 28, wherein the method implemented by the computer furtherincludes the steps of: scrambling the source data prior to thegenerating step.